1. Field of the Invention
The present invention relates to image processing. More particularly, the present invention relates to processing multiple frames of image data from a scene.
2. Background Information
Known approaches seek to identify moving objects from background clutter given multiple frames of imagery obtained from a scene. One aspect of known approaches is to align (register) a first image to a second image and to difference the registered image and the second image. The resulting difference image can then be analyzed for moving objects (targets).
The Fried patent (U.S. Pat. No. 4,639,774) discloses a moving target indication system comprising a scanning detector for rapidly scanning a field of view and an electronic apparatus for processing detector signals from a first scan and from a second scan to determine an amount of misalignment between frames of such scans. A corrective signal is generated and applied to an adjustment apparatus to correct the misalignment between frames of imagery to insure that frames of succeeding scans are aligned with frames from previous scans. Frame-to-frame differencing can then be performed on registered images.
The Lo et al. patent (U.S. Pat. No. 4,937,878) discloses an approach for detecting moving objects silhouetted against background clutter. A correlation subsystem is used to register the background of a current image frame with an image frame taken two time periods earlier. A first difference image is generated by subtracting the registered images, and the first difference image is low-pass filtered and thresholded. A second difference image is generated between the current image frame and another image frame taken at a different subsequent time period. The second difference image is likewise filtered and thresholded. The first and second difference images are logically ANDed, and the resulting image is analyzed for candidate moving objects.
The Markandey patent (U.S. Pat. No. 5,680,487) discloses an approach for determining optical flow between first and second images. First and second multi-resolution images are generated from first and second images, respectively, such that each multi-resolution image has a plurality of levels of resolution. A multi-resolution optical flow field is initialized at a first one of the resolution levels. At each resolution level higher than the first resolution level, a residual optical flow field is determined at the higher resolution level. The multi-resolution optical flow field is updated by adding the residual optical flow field. Determining the residual optical flow field comprises the steps of expanding the multi-resolution optical flow field from a lower resolution level to the higher resolution level, generating a registered image at the higher resolution level by registering the first multi-resolution image to the second multi-resolution image at the higher resolution level in response to the multi-resolution optical flow field, and determining an optical flow field between the registered image and the first multi-resolution image at the higher resolution level. The optical flow determination can be based upon brightness, gradient constancy assumptions, and correlation of Fourier transform techniques.